U.S. airlines’ on-time performance declined dramatically in December compared to the previous month, but improved slightly over a year earlier, the Transportation Department said Monday. The carriers blamed heavy snowstorms around the Christmas holiday and aviation system issues for contributing to some delays.Airlines had a significantly higher domestic flight cancellation rate and a higher mishandled baggage rate in the final month of 2008 compared to November. Even complaints about airline service were up in December compared to November.